I am sure there must be tests, but they would be designed for the use of medical practitioners and are best interpreted by them.

In fact, my suggestion is to make no attempt to self diagnose (or in this case diagnose another) without the input of a professional. The best thing you can do for them is to encourage them to seek professional help - starting with a family doctor or the prescribing individual responsible.

There are multiple reasons why one can lose cognitive function. Some of those reasons are medical in origin. If it's a woman you speak of, is she approaching time of life? And then yes, it could be medication.

But still, it should be a doctor that you engage to get to the bottom of this.